The problem is that your table is about 520px wide with the images and you also have margins on the side which make it larger than 600px. When the browser resizes and meets the table then the other elements keep on re-sizing smaller such as your h1 but the table is held wide.
Mozilla still collapses the background but behind the table.
You could give your table a width and take out the margins to make it look ok at 600pxl
Alternatively just float your images left (not in a table) and then the page can resize at leisure. You should try and make your images the same size so that it looks neater.
border: 1px dashed;
margin: 10px 10px;
padding: 0px 0px 0px 10px;